Afactorymachinecutsmetalwireintostrips.Thelengthof thestripsisknown tofollowanormaldistributionwithmeanof22cmandstanda rddeviationofσ. Thestripsarepackagedandsoldingroupsof12.Thecompany claimsthat90%of thesestripsarepackagedwithanaveragelengthwithin21c mand23cm.Whatis thevalueofσ?

Use the Z-score table in your text book or available on line to determine the value of z that corresponds to an area under the normal probability curve of 90%, i.e the area is 0.45 to the left of the mean and 0.45 to the right. Then you can calculate sigma from

z=\frac {x - \mu}{\sigma} = \frac {23 cm - 22cm}{\sigma}